Thank you guys - my brain has just clicked into gear.
All started because I needed to fly under an airway which specified +3000'. I questioned whether to use a local airfield QNH or the Barnsley and looked to the half mil map to see if it told me
Of course 3000' is a fixed point in the sky 3000' above the level of the sea so it doesn't matter what you use to measure this, even a piece of string! Pressure doesn't come into it at all.
So if you want to find this point you use the best instrument you can (the altimeter) and you use the pressure setting most applicable to the bit of sky you are in.
So local airfield QNH is as good as you can get here.
Don't you hate it when you demonstrate your dumbness in public
